Patent Literature 1 discloses an art to analyze a pattern of a change in an air pressure of a tire when the detected change in the air pressure is caused by a deflation of the tire and a rotation of the tire while an ignition (IG) of a vehicle is in the off state. In the art of Patent Literature 1, when the result of the analysis indicates that a change in the air pressure is higher than a threshold, it is determined that the change is intentionally caused by a thief of the vehicle. Further, a phone equipped to the vehicle is activated by this determination, and alerts the theft of the vehicle to a mobile phone carried by the vehicle owner via a service center.
However, a passenger such as a driver usually turns off the IG before exiting the vehicle. Therefore, a weight change due to an exit from the vehicle after the turning off of the IG, an impact due to opening and closing of the doors, and a weight change due to loading and unloading of baggage may cause the air pressure to be changed over a threshold. This air pressure change may be erroneously determined as the intentional theft of the vehicle. In such a case, the passenger may have to handle an alert activated by the erroneous determination.